1. Field of the Invention
The present invention relates to the field of photography and more specifically, to multi-element lenses for use in photographic apparatus.
2. Description of the Prior Art
The present invention addresses itself to the problem of economically manufacturing large numbers of precision multi-element objective lenses.
The degree to which a multi-element lens approaches its design specification is, to a large extent, determined by the precision with which the individual lens elements are aligned or optically centered with respect to one another in a lens mount, assuming of course that the individual lens elements are within their design tolerances.
The opposing curved surfaces of a simple lens element may be thought of as sections of two spheres. The optical axis of such a lens lies along an imaginery line joining the centers of the two spheres. The geometrical axis, on the other hand, is an axis of symmetry of the circular peripheral edge of the lens element. When the optic and geometrical axes coincide, the lens is said to be "centered".
Traditionally, a lens is centered by mounting it in an edge grinding machine so that it rotates about the optical axis. The peripheral edge is then ground to a predetermined diameter thereby making the optic and geometrical axes coincidental. A plurality of centered elements are mounted in a cylindrical hollow lens mounting tube to form the multi-element lens assembly.
Alignment of the lens elements, with respect to one another, is accomplished mechanically. The interior surface of the mounting tube acts as a common reference surface against which the peripheral edge of each lens element abuts. As would be expected, the precision with which the assembled multi-element lens is centered depends on the tolerances to which the interior surface of the mounting tube and the diameters of the individual lens elements are machined.
An example of a multi-element, adjustable focus, lens assembly constructed in the above-described traditional manner is disclosed in a now abandoned application Ser. No. 101,745, filed on Dec. 28, 1970 by Rosario J. Messina, said abandoned application being assigned to the same assignee as the present application.
Grinding the peripheral edges of the individual lens element is an expensive operation. When multi-element lenses are manufactured in large volumes, it represents a significant percentage of the total manufacturing cost, not only because of the need for skilled personnel, but also the time it takes to accurately center each of the lens elements on its optical axis in the grinding machine.
An alternative to edge grinding is disclosed in U.S. Pat. Nos. 2,304,984; 2,388,893; 2,266,169; 2,341,364 and 2,245,257. A simple uncentered lens element is held between a pair of opposed zonal clamps axially disposed in the cavity of a cylindrical mold. The zonal clamps each include a hemispherical cup bounded by a low friction annular ring which engages one of the spherical curved surfaces of the lens element. When the lens element is held between the opposed rings, it slides transversely to the axial centerline of the clamps until the optical axis of the lens element coincides with the centerline of the clamps and the geometrical axis of the cylindrical mold. Plastic is then injected around the periphery of the lens element to form a rim which compensates for difference in radial measurements between the peripheral edge of the lens element and the optic and geometric axes. The lens element is now "centered" with respect to the outside edge of the plastic rim and this "lens cell" may be inserted into a lens mounting tube to form a multi-element lens in cooperation with other lens cells.
In general, the previously mentioned patents teach the concept of molding a compensating rim onto a glass or plastic lens element. The molded lens cells are then individually inserted into a metal or molded plastic lens mounting tube, in the conventional manner, to form a multi-element lens. U.S. Pat. No. 2,266,169 carries this process one step further by disclosing a viewfinder in which two lens elements are held in optically centered alignment and predetermined spaced relation by two pair of zonal clamps. Plastic is then injected to form three sides of a square lens mounting tube which has sections integrally formed therewith for retaining the two aligned lens elements. The fourth side of the tube is left open to permit the removal of zonal clamps disposed between the forward and rear lens elements. A cover for the open side of the lens mounting tube is molded separately and is cemented in place after removal of the clamps.
The prior art molded plastic lens mounting systems are mainly directed to fixed focus multi-element lenses. That is, the plastic rimmed lens cells are permanently mounted in fixed spacial relation with respect to one another to form the multi-element lens. Also the prior art systems feature rather large air spaces between adjacent lens elements and the front to rear thickness of the plastic centering rims do not pose a problem such as when the design of the multi-element lens requires that at least two of the lenses be separated by a minimum air gap. Also, the lens elements are not integrally molded into a complete encircling mounting tube thus requiring additional molding and assembly operations.